The present invention relates to a magnetic sensor for use in magnetic encoders, tacho-generators, card readers, etc., and more particularly to a magnetoresistance-type magnetic sensor and a card reader comprising such a magnetoresistance-type magnetic sensor.
A magnetoresistance-type magnetic sensor (hereinafter referred to simply as "magnetic sensor") is an element for converting a periodic magnetic field variation to the variation of output voltage by utilizing the phenomenon that a magnetoresistive material's resistance varies depending upon the intensity of an external magnetic field. For instance, see Japanese Patent Publication No. 57-5067.
As rotatable means for generating a periodic change of a magnetic field, magnetic discs such as shown in FIG. 14 and magnetic drums such as shown in FIG. 15 are already known. The magnetic disc 2 has a magnet belt having a larger number of magnetic poles on its surface and is rotatable by a center shaft 3. The magnetic poles are opposing to a sensor means 1 with an extremely small distance to detect the variation of a magnetic field generated from the disc surface. On the other hand, the magnetic drum 50 has a magnet belt having a large number of magnetic poles on its cylindrical side surface and is rotatable by a center shaft 3. Similarly, the magnetic poles are opposing to a sensor means 1 with an extremely small distance to detect the variation of a magnetic field generated from the drum surface.
Since the magnetic drum-type rotatable means has a large thickness, it is not suitable for miniaturization. On the other hand, the magnetic disc-type rotatable means may be extremely thin because its magnetic field-generating portion is located on one of the flat, circular surfaces of the disc.
In the magnetic sensor of the above structure as shown in FIG. 14, however, there is a problem that the slightest inclination of a rotation axis of the magnetic disc 2 causes the wobbling of the magnetic disc 2, which leads to a relatively large variation in the distance between the disc surface and the sensor means 1 opposing thereto. This variation is exemplified in FIGS. 13 A and B in an exaggerated manner. That is, in FIG. 13 A, the distance between the disc surface and the sensor means 1 is small, while that distance is large in FIG. 13 B. Accordingly, an output voltage amplitude is relatively large in the case of FIG. 13 A, while it is relatively small in the case of FIG. 13 B. The cases of FIGS. 13 A and B take place alternatingly as the magnetic disc rotates, resulting in the periodic variation of an output voltage amplitude. Therefore, it is difficult to provide an output voltage having a constant amplitude with the magnetic disc-type rotatable means.
In addition, since the sensor means provided with the sensor element is positioned as close to the magnetic disc as 10 .mu.m-3 mm, particularly 10-200 .mu.m or so, the terminals and solder portions on the sensor support, which have a relatively larger thickness, are likely to collide with the disc surface, even if the magnetic disc undergoes the slightest wobbling. Accordingly, it is necessary to make sure that the terminals and solder portions of the sensor element would never collide the disc surface even if there is some wobbling in the magnetic disc.
Further, the magnetic disc-type magnetic sensor is suitable for use in a card reader in which a rotary means rotatable synchronously with the card provides a signal showing the position of the card.